Modern optical communications networks are universally used to interconnect distant, regional, and metropolitan communications hubs for directing numerous diverse streams of telephony, digital video, internet, and other types of digital data. The means for most efficiently and economically managing the ever-increasing capacity and speed demands on these networks, many communications channels are aggregated into streams each carrying up to 10 gigabits per second, presently emerging 40 and 100 gigabits per second, and future prospects for multiple hundreds of gigabits per second per aggregated data stream. Dozens of these data streams are transmitted simultaneously through each fiber in the network utilizing wavelength-division multiplexing (WDM) where each stream is carried by an optical signal having an optical wavelength slightly different but fully distinguishable from all the other wavelengths for the other streams in the fiber. These optical streams are routinely combined and separated as appropriate by various well-known optical filter components at each end of the optical fiber link.
These optical networks include many locations where optical fibers intersect at ‘nodes’. These nodes are in many ways analogous to the intersections of a complex highway system. Much traffic comes to the node along each of the fibers, but not all the traffic on any fiber is necessarily bound for the same destination. Some of the traffic may be bound for destinations local to the node, there may be new traffic originating local to the node, and other traffic may need to be independently rerouted among the various outbound fibers from the node. Effecting the necessary reconfiguration of traffic at these nodes is provided by switches.
Until recently, the primary means to provide such switching would be electronic. To accomplish this, every wavelength in each fiber would be separated to individual physical channels, and then the data in each of those wavelengths would be converted by an optical receiver into binary electrical data. Once all the data is in electrical form it can be piped into an electronic switching matrix in any of numerous possible configurations, and reorganized into appropriate groupings on multiple output channels. Then the data in each output channel is converted back to optical by an optical transmitter at each output having a specific predetermined wavelength and the data streams on distinct wavelengths bound for each output fiber are remultiplexed and inserted into that fiber. There may also be input and output data streams associated with local traffic that can be integrated with the data passing through the node using additional ports on the switching matrix. At the data rates used in each wavelength, electro-optic receivers and transmitters are relatively expensive, bulky, and power hungry as compared to purely optical dispatch. Also, within an electrical switch matrix, electrical power is required to push each and every bit of data through the matrix, and there may be hundreds of billions or trillions of bits moving through the matrix every second. In principle, electronic switching can provide the ultimate flexibility in reconfiguring, formatting, synchronizing, and otherwise optimizing the presentation of the data before sending it on its way. However, for the amount of data passing through a modern node, it is far and away simply impractical to switch everything electronically, and the economics of providing the fundamental hardware is also unsupportable. Furthermore, the bandwidth passing through the nodes is only expected to increase with time.
Optical switching technology has been emerging to complement the electronic switching in concurrence with, and in fact enabling the increase in bandwidth of the data passing through the nodes. Optical switching generally treats each wavelength as a cohesive unit and passes each wavelength transparently to its destination within the node, either an output fiber or a wavelength channel associated with local traffic. The transparent optical switch effectively establishes a physical path for the light at the specified wavelength on the specified input fiber to be passed linearly and directly to the desired output fiber or local port. Such a switch essentially passes any optical data regardless of format or content as long as it is within the optical wavelength range specified for that optical channel. Since the optical switch cannot modify the detailed data within the optical wavelength, it is not as flexible as an electronic switch. But more significantly, the power required to switch the data for that wavelength is merely the amount of power needed to establish and maintain the optical path through the switch, which is generally orders of magnitude less than required for electronically switching the same data. As power consumption is often the limiting factor for the bandwidth that can be managed by a node, optical switching is not merely a convenience of remote configuration, but clearly enables the current and future performance levels of optical networks.